The dynamic stress-intensity factor about the tip of a rapidly moving crack ( K D ) in polymethylmethacrylate (PMMA) has been measured, using ultra-high speed photographic techniques to record the photoelastic patterns produced around the crack tip. From the same photographs, the velocity of the moving cracks was determined. A gross roughening of the fracture surface was found to appear at crack speeds of 1,300 ft sec −1 (396 m sec −1). From the geometry of the surface markings associated with the roughening, a dynamic value of Young's Modulus for PMMA was obtained over the velocity range 1,300–2,300 ft sec −1 (400–700 m sec −1). These dynamic values of Young's Modulus were used to interpret the variation of the stress intensity factor, K D , with crack velocity, and its relation to fracture morphology.